Spintronic devices are electronic circuits that could use both the charge and spin of charge carriers, most typically electrons, to transmit, store and process information. There are many applications for spintronic devices. Some examples of spintronic devices are the family of devices which exploit giant magnetoresistance (GMR). Spintronic devices may be used, for example, in magnetic field sensors, magnetic memories, spin-based transistors, semiconductor quantum interference devices based on electron spin, classical and quantum computers, heads for reading data from magnetic storage media or the like.
An impediment to the availability of commercially useful spintronic devices is that it has so far not been practical to generate strongly spin-polarized currents in semiconductor materials (often referred to as “spin injection”). Spin-polarized currents of electrons can be generated in ferromagnetic materials because the magnetic field of the material (augmented by many-body effects) interacts with the spins of electrons. Thus, the spin-up and spin-down states of electrons in a ferromagnetic material have different energy levels. The majority of electrons are in states such that their spins are aligned with the local magnetic field of the material. In such materials, electrons occupy the majority and minority spin states asymmetrically.
It is possible, in principle, to take advantage of this asymmetry to create a spin-polarized current in a non-ferromagnetic material (e.g., semiconductor) by creating a spin-polarized electronic current in a ferromagnetic material and causing the current to flow from the ferromagnetic material into an adjacent non-ferromagnetic material by way of an interface. For example, if a layer of ferromagnetic metal is bonded to a piece of silicon, the electrons can be made to flow from the magnet into silicon by applying a voltage. Unfortunately, the electrons lose their polarization as they cross the interface between the two materials thanks to an “impedance mismatch” between the metal and the semiconductor. As a result, the efficiency of spin injection from ferromagnetic metals into semiconductors has been very low.
This problem has been circumvented in semiconductor materials, such as gallium arsenide, by allowing the spin-polarized electrons to “tunnel” across the interface, thus avoiding the impedance mismatch. However, such a process requires a very thin and abrupt interface between the metal and the semiconductor, which cannot currently be achieved when growing layers of ferromagnetic metal on semiconductor material, such as silicon.